A number of prior art processes have been proposed for the preparation of oxalate esters by the oxidative carbonylation of alcohols in the presence of metal salt catalysts, dehydrating agents and ferric or cupric redox agents in solution.
The present invention is directed to an improved process for the preparation of oxalate esters in high yield and avoiding problems associated with the prior art processes of carbonylating alcohols. More particularly, the present process relates to the synthesis of oxalates by reacting carbon monoxide, an alcohol, and oxygen under elevated temperature and pressure conditions in the presence of a catalytic mixture of materials comprising (1) a palladium, platinum, cadmium, rhodium, or copper salt compound, (2) at least a catalytic amount of an amine base, (3) a copper (I), copper (II), iron (II) or iron (III) oxidant salt in catalytic quantities and (4) catalytic amounts of an ammonium or substituted ammonium salt compound. Ligands or a coordination complex of the metal salt compounds may also be employed.
U.S. Pat. No. 3,393,136 describes a process for the preparation of oxalates by contacting carbon monoxide at superatmospheric pressure, with a saturated monohydric alcohol solution of a platinum group metal salt and a soluble ferric or cupric salt (redox agent) while maintaining the salts in a highly oxidized state by the simultaneous introduction of oxygen or the application of a direct current electrical potential to the reaction zone. Water scavengers or dehydrating agents such as alkyl orthoformic acid esters must be added to the liquid phase to prevent the accumulation of water.
In a recent article by Donald M. Fenton and Paul J. Steinwand, Journal of Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a general mechanism for the oxidative carbonylation of alcohols to yield dialkyl oxalates using a palladium redox system, oxygen and dehydrating agents has been proposed. In the absence of the necessary dehydrating agent, a large amount of carbon dioxide is formed and oxalates are not produced. The necessity of the iron or copper redox system during the oxalate synthesis is emphasized.
A recent West German Pat. No. 2,213,435 discloses a method for the synthesis of oxalic acid and oxalate esters using water and alcohol respectively. A platinum group metal salt, a salt of a metal more electropositive than the platinum group metal, e.g. copper (II) chloride and an alkali metal salt such as lithium chloride comprise the catalyst. Oxygen in stoichiometric amounts was employed as the oxidant. A disadvantage of such reaction is that explosive mixtures of oxygen and carbon monoxide are necessary to effect reaction. Alcohol conversions of less than 5 percent are obtained. Under non-explosive conditions only trace amounts of oxalate can be obtained.
U.S. Pat. No. 3,994,960 describes a process for the production of dialkyl oxalates by reacting an aliphatic alcohol with CO and oxygen under pressure in the presence of a catalyst of a mixture of a salt of a metal from the platinum group and a salt of copper or iron and a reaction accelerator including nitrates, sulfates, bicarbonates, carbonates, tertiary amines and hydroxides and carboxylates of alkali metals and alkaline earth metals, pyridine, quinoline, urea and thiourea. Conversion of the alcohol employed to the dialkyl oxalates in such process is low, generally less than 9 mole percent.
In a process similar to that of U.S. Pat. No. 3,994,960 above, West German Offenlegungsschrift No. 2,601,139 shows the production of oxalic acid or its alkyl esters by reacting aliphatic alcohols or water with oxygen and carbon monoxide in the presence of palladium salts, redox salts and a base of amine or ammonia.
Many important commercial applications have been developed for the oxalate products of this invention, for example, cellulose ether or ester and resin solvents, as dye intermediates and the preparation of pharmaceuticals.
The process of the present invention provides an improved process for the oxidative carbonylation of an alcohol to produce an oxalate ester. Thus, there is provided a high conversion of the alcohol employed and excellent yield selectivity to the oxalate ester. Carbonate esters, carbon dioxide and alkyl formate as well as other side products commonly associated with such reactions are suppressed by employing as part of the catalyst the ammonium or substituted ammonium salt compound, i.e., the amine salt, in conjunction with the amine, and the palladium, rhodium, platinum, copper or cadmium metal salt compound and the copper (I), copper (II), iron (II) or iron (III) oxidant salt compound and alternatively a ligand or coordination complex, such as lithium iodide, and by initially charging the reactants in an anhydrous condition. The addition and presence of the amine salt in the catalytic mixture of the invention has been found to be necessary in order to achieve the increased selectivity and yield of oxalate ester not obtained by prior art processes. The addition of the amine salt maintains the proton acidity of the reaction system.
Other advantages of the present invention, as compared to known prior art processes for the production of oxalate esters are (1) elimination of the use of expensive water scavengers, by substantially inhibiting side reactions resulting from the formation of water in the reaction system, e.g., alkyloxalate anion (ROOCCOO--), oxalate dianion (C.sub.2 O.sub.4 .dbd.), and carbonate anion (CO.sub.3 .dbd.), (2) ease of recovery and reoxidation and recycle of reduced oxidant salts and regeneration and recycle of the metal salt compounds, (3) avoiding the use of large amounts of corrosive halogen ions and in some cases, elimination of halogen ion concentrations, (4) elimination of hazardous operational conditions by avoiding explosive mixtures of oxygen and carbon monoxide, (5) the ability to employ the more readily available copper, or cadmium salt compounds in place of the more expensive platinum group metals or metal salt compounds, and (6) the ability to employ ammonia, or a primary or secondary as well as a tertiary amine base.